Austenitic stainless steel

ABSTRACT

This invention concerns an austenitic stainless steel composed principally of manganese, chromium and nickel and which contains small, but in some cases significant, amounts of carbon, silicon, copper and nitrogen. The materials are combined in critical proportions insuring that the amounts of martensite and delta ferrite present are within controlled maximums and insuring that good hot workability is maintained.

United States Patent Continuation of application Ser. No. 315,957, Oct. 14, 1963, now abandoned.

AUSTENITIC STAINLESS STEEL Primary Examiner-Hyland Bizot Attorneys-Richard A. Speer, Vincent G. Gioia and James A.

Berneburg ABSTRACT: This invention concerns an austenitic stainless steel composed principally of manganese, chromium and 4 Claims 3 Drawing Figs nickel and which contains small, but in some cases significant, ILLS. Cl 75/125, amounts of carbon,'silicon, copper and nitrogen. The materi- 75/128 A als are combined in critical proportions insuring that the Int. Cl C22c 39/54 amounts of martensite and delta ferrite present are within con- Field of Search 75/125, trolled maximums and insuring that good hot workability is 126.5 maintained.

HMQO HM85 I HM75 POOR HOT ROLLABILITY H0900 HQ7| H874 aass? X 6 I 65 3 {W66 H575 HQ 3 H0901; GOOD HOT ROLLABILITY '3 HQQOA HQ'OOBB MRGBz HR4 HRIS Degree of Cracking During Hot Rolling of lngot 0 None HQIOOAJ HRSA X Light 8 A Light to Severe V Severe El Very Severe o 2 4 e e l0 l2 PATENTEDUEI 26 I9?! SHEET 2 OF 2 FIG.3

HEAT 348695 HEAT 55745 /o ELONGATION i 009 mmwEw M55 INVENTOR. THoMAsHMcCuNN ATTORNEY AUSTENITIC STAINLESS STEEL Of the many different types of stainless steels, the A181 200 and 300 series are the austenitic stainless steels. The compositions of these alloys are balanced in such a way that the steels are substantially fully austenitic after normal mill processing, and they are normally sold in this austenitic condition. The steels of the AISl 200 Series are basically chromium-nickelmanganese stainless steels, and those of the 300 Series are basically chromium-nickel stainless steels. Although these steels in their normal as-processed condition are fully austenitic, many of the grades are unstable in that they will form appreciable amounts of martensite when they are cold worked such as by deep drawing or cold heading. For example, AISl Type 201 and A151 Type 301 stainless steels may form as much as 50 percent or more martensite upon severe deep drawing. The propensity to form martensite upon cold working can be reduced or even eliminated by increasing the alloy content, especially the nickel, as in AISl Type 304 and A181 Type 305, and such grades form little or no martensite, even when severely cold worked, hence making them stable alloys. However, AlSl Types 304 and 305 stainless steels contain appreciable amounts of chromium and nickel, which are particularly expensive, especially nickel, and therefore it is costly to provide this stability.

It is therefore a principal object of this invention to provide a relatively inexpensive, stable, austenitic stainless steel which forms little martensite upon severe cold working.

A related object of this invention is to provide a stable, austenitic stainless steel having a balanced composition which will permit conventional mill processing of the alloy, yet which will be relatively stable, even upon cold working.

A further object of this invention is to provide a stainless steel of balanced composition which is suitable for deep drawing and cold heading.

Yet another object of this invention is to provide an improved valve steel, and an improved deep-drawing steel.

These and other objects, together with a fuller understanding of the invention, will become apparent from the following description when taken in conjunction with the accompanying drawings in which:

FIG. 1 is a graphical representation of the relationship between manganese and copper contents of an alloy according to this invention, showing how this relationship affects the alloys hot rollability;

FIG. 2 is a graphical representation of the balance of elements required for controlling the delta ferrite-forming tendencies of an alloy according to this invention; and

FIG. 3 shows a typical true stress-elongation curve for an alloy according to this invention compared to a typical true stress-elongation curve of A181 Type 304 stainless steel.

The forming characteristics of austenitic stainless steels de pend upon the rate of work hardening which, in turn, is controlled primarily by the composition of the alloy. The rate of work hardening is determined by the following three, factors, all of which are controlled by the composition: (1) the workhardening characteristics of the austenite itself; (2) the amount of martensite formed during deformation; and (3) the work-hardening characteristics of the martensite once it starts to form and deformation is continued. Of these three factors, the one that has by far the greatest effect on the work hardening, or rate of work hardening, is the amount of martensite formed during deformation. The difference in the rate of work hardening between Types 201 and 501 stainless steels on one hand, and Type 304 on the other, is primarily a function of the different amounts of martensite formed in each during cold deformation. Types 201 and 301, which may form percent or more martensite, are thus unstable in this respect, whereas Type 304, which forms less than 10 percent martensite, is quite stable in this respect. Generally speaking, an austenitic stainless steel can be considered stable if it forms less than about 10 percent martensite upon heavy cold deformation, and considered unstable if it forms about 10 percent or more martensite. THe 10 percent limitation is imposed from practical considerations, inasmuch as if there is a greater amount than 10 percent martensite formed, deep-drawing operations are less desirable since cracking of the workpiece and/or excessive die wear tends to occur.

1 have found that by properly balancing the elements, a chromium-manganese-nickel-copper austenitic stainless steel can be provided which has a low work-hardening rate similar to that of Type 304 stainless steel, and in which the alloying elements are substantially less expensive than those in the Type 304 stainless steels and comparable in price to those in Type 201 and Type 301 stainless steels. The alloy consists essentially of up to about 0.12% carbon, from about 5 to about 8.5% manganese, up to about 2% silicon, from about 15 to about 17.5% chromium, from about 3 to about 6.5% nickel, from about 0.75 to about 2.5% copper, up to about 0.05% nitrogen, and the remainder iron and incidental impurities. Within this range for each of the elements it is necessary to control these elements so that the martensite-forming characteristic of the alloy is less than about 10% according to the formula It is to be understood that this formula describes the martensite-forming characteristic of the alloy in terms of the amount of martensite which will be formed upon severe cold deformation, and that the alloy in the normal as-processed condition is substantially completely austenitic. Several examples of different alloys are given in table 1 below.

TABLE I Composition, 11" value. and percent martensite of specific stainless steels Murtenslte Percent. Percent Heat. No. 0 Mn Si Cr Ni Cu N 11" ic served culnti'd Table l-Continued Martenslte Percent Percent Heat No. Mn Si Cr Ni Cu N 11" o c served culated Of these examples in table I, Heats HN85A, HN85B, HN86A, GW69 and HN86B are all within the scope of this invention, as are heats 55688, 55739, 55743, 55744 and 55745. Heats HN84A, HN84B, HN87A, HN89A, HN89B, HF64, 0W5, HG31 and H635, although otherwise within the broad limits of this invention, contain less copper than the minimum required. Heat HN88, although within the composition limits, has a martensite-forming potential, calculated according to the formula shown above, of 18.8 percent, and hence is outside the scope of the invention. Heats HN87B, HF65, HF58, HF57, GW36 and H033 each contain one or more elements outside the range encompassed by this invention in addition to having less than 0.75% copper, and hence are also outside the scope of this invention. Heats HN82A, HN82B, HN82C, HL and HC29 show the effect of varying amounts of copper added to Type 301 stainless steel and are not within the scope of the invention. Heats 348639 and 348695 are mill heats of conventional Type 304 stainless steel, and Heats 55315 and 21310 are mill heats of conventional Type 305 stainless steel. The n" value listed in table I is the mean slope of the true stress-elongation diagram between 25 and 50% elongation, two typical stress-elongation diagrams being shown in FIG. 3. This n" valuegives indication of the work hardening characteristics of the material; the higher the n value, the greater the rate of work hardening. The percent martensite observed is the amount of martensite as determined by measurements made with a MAGNE-GAGE Tester taken about 1 inch from the fracture of tensile test specimens. MAGNE-GAGE is a trademark of American instrument Company, Silver Spring, Md. This value, although not agreeing exactly with the formula, is very close thereto and has a high degree of correlation therewith. The formula given above was used to calculate the predictable amount of martensite that would form in each of these specimens, and as can be seen from the table, in each case agrees quite well with the observed values for alloys within the composition range contemplated by this invention. It should be noted, however, that the formula for calculating the percent martensite loses its high degree of correlation outside the broad limits of this invention, especially when the nickel content is quite high as in Type 305 stainless steel, and hence should be used very cautiously or not at all to predict the percent martensite which will be formed in alloys outside the scope of this invention. A comparison of the n" value and percent martensite formed in these alloys shows that conventional Type 304 is comparable in work-hardening characteristics to the alloy of this invention.

Table 11 below shows the tensile properties of various alloys listed in table I. This table shows that alloys balanced according to this invention are admirably suited for deep drawing. Table II also shows MAGNE-GAGE measurements of the percent martensite formed upon SO-percent cold reduction. These values correlate quite well with the measured percentage of martensite in the tensilespeciments and with the calculated values from the formula and shown in table I.

TABLE II Annealed cold reduction 0.2% Y.S., U.T.S., Elong., Hard- U.T.S,, I-Iard- Martcnsitc, Heat No. p.s.i. p.s.i. percent ness, Rb p.s.i. ncss, Rn percent It is well known that it is necessary to provide the proper balance of elements in stainless steels so that excessive delta ferrite is not formed. In most conventional size ingots, if more than about 10 percent delta ferrite is present during rolling, the resultant product will tend to have slivers, hot tears and be prone to cracking; hence extra precautions and special treatments are necessary if more than about 10 percent delta ferrite is present. These extra precautions and special treatments in many instances are costly, and hence should be avoided if possible. Thus, it has been found to be necessary in the present invention to balance the elements in the composition so that the delta ferrite-formin g potential is less than 10, according to the formula delta ferrite-forming potential The line described by this formula, shown graphically in FIG. 2, approximates 10 percent delta ferrite in normal size ingots during the initial stage of hot rolling. When the balance of the elements falls to the lower right of the line, excessive delta ferrite is formed, and hence the alloy is outside the scope of this invention, but when the value falls to the upper left of the line and the elements are otherwise within the composition range, the alloy is within the scope of this invention with respect to delta ferrite-forming potential. It is to be understood that the delta ferrite will disappear, or revert to austenite, during hot rolling so that the final product of rolling will be substantially free of delta ferrite.

It has also been found that there is a very definite relationship between the amount of manganese and copper present and the hot shortness of the alloy. In the higher ranges of manganese it is not possible to use as much copper as in the lower ranges of manganese. FIG. 1 graphically represents the balance between manganese and copper which must be maintained in order to prevent hot shortness, and consequently prevent poor hot rollability. If the balance of manganese and copper lies above the line shown in FIG. 1, the alloy cannot be properly rolled, and hence is outside the scope of this invention. The balance of copper and manganese must fall below the line in order for the alloy to be within the scope of this invention. The various heats listed in table III below are plotted on the graph of FIG. 1. Even slight cracking during hot rolling must be avoided, hence only those alloys which exhibit no cracking are acceptable. The limiting factor shown by the line in FIG. 1 can be described by the expression Max. Cu does not exceed 3.850.18(% Mn) TABLE III Compositions of heats to determine hot rollability characteristics Relative degree of Heat No. Mn Si Cr Ni 011 N cracking HM62 n 060 6. 86 47 16. 12 5. 78 2. 11 None. H1166. .062 6. 86 .43 16.08 5. 84 2. 73 Light.

6. 00 53 16. 90 5.86 3. 52 Very severe. 4. 00 55 16. 14 6. 90 3. 57 Severe. 6.00 .54 16. 23 8. 30 3. 54 Very severe. 6. 02 .37 16. 19 5.01 1. 98 0. 01 .37 16. 58 5. 14 2. 06 11.75 .41 15. 92 5.17 2.05 9. 01 .37 16. 58 5. 14 1. 11. 75 .41 15. 92 5. 17 1. 00 6.02 .37 16.19 5. 01 2. 48 6. 02 .37 16. 19 5.01 2. 94 050 Light. 3. 48 .63 16.60 6. 66 2. 56 027 None. 6. 57 54 16. 69 8. 2. 97 036 Severe. 8. 85 .51 1G. 64 8. 54 2. 03 045 None. 9. 37 5'2 14. 60 5. 22 2. 03 034 D0. 3. 07 .56 16. 67 5. 70 2. 86 .030 Do. 8. 00 .65 16. 55 5. 45 2. 9-1 054 Light-severe. 10.20 .64 16.35 5. 43 2. 95 .057 Severe.

With respect to the composition limits, the chromium content cannot be below 15% since with less than this amount excessive amounts of the other alloying elements would be also lead to hot shortness problems in the balance of copper and manganese if these elements are increased to prevent the formation of delta ferrite. Hence, the broad limits for chromiurn are about 15 percent to about 17.5 percent. If there is less than 3 percent nickel, the manganese and/or copper contents would have to be increased to obtain stability with respect to formation of martensite and to prevent formation of excessive delta ferrite. This would tend to make the alloy hot short if enough manganese and/or copper were added to compensate for the reduced amount of nickel. More than about 6.5 percent nickel adds substantially to the cost of the alloy. There must be at least 5 percent manganese since less than this would require excessive amounts of nickel, adding greatly to the cost of the alloy, or excessive copper which would tend to make the alloy hot short if the desired stability is to be obtained. There cannot be more than 8.5 percent manganese since the amount of copper that can be used would be correspondingly reduced because of hot shortness problems. With less than about 0.75 percent copper, excessive amounts of nickel and/or manganese are required to effect the stability of the alloy since copper is almost twice as effective as nickel and more than three times as effective as manganese, as shown by the formula, in stabilizing the alloy against formation of martensite during deformation. With more than 2.5 percent copper, the alloy tends to be hot short unless the manganese content is maintained low, and the copper has a lesser effect on the suppression of delta ferrite, as shown by the formula, than either manganese or nickel. Although carbon and nitrogen contribute substantially to providing stability and to the suppressing of the formation of delta ferrite, when these amounts exceed about 0.12% and about 0.05% respectively, they increase the yield strength and tensile strength of the alloy to a level which makes the alloy unsuitable for deep drawing, cold heading and other similar forming operations.

With an alloy falling within the broad limits described above and wherein the elements are controlled so that the martensite-forming potential is less than 10%, the delta ferriteforming potential is less than 10 percent and the copper does not exceed 3.85-O.l8(% Mn), an easily workable, relatively inexpensive austenitic stainless steel having excellent deepdrawing characteristics is produced.

Although the broader limits of the alloy have been previously described, it has been found that it is preferable tocon; trol the elements within a somewhat narrower melting range in order to afford maximum economy of elements and still provide optimum characteristics in the alloy. Within this narrower melting range it is easier to achieve an economic balance of the elements to utilize these various characteristics. This melting range is as follows: about 0.07% max. carbon, from about 5.5 to about 6.5% manganese, up to about 2% silicon, from about 16.1 to about 16.6% chromium, from about 5 to about 5.5% nickel, from about 1.5 to about 2% copper, up to about 0.05% nitrogen and the remainder iron with incidental impurities. Within this melting range, however, it is still essential that the martensite-forming potential, the delta ferrite-forming potential and the copper-manganese balance all be maintained according to the formule given in order for the alloy to be within the scope of this invention.

Table IV below shows the results of various tests to determine the resistance to corrosion of alloys according to this invention. Heats 55688, 55744 and 55745, the compositions of which are given in table I, were compared to a conventional AISI Type 201 stainless steel (Heat 338858) having a composition of 0.098% carbon, 6.48% manganese, 0.028% phosphorus, 0.006% sulfur, 0.57% silicon, 16.60% chromium 5.40% nickel, 0.17% copper, 0.1 1% nitrogen and the balance iron. The test results in table IV indicate that alloys according to this invention are comparable in corrosion resistance to Type 201 as measured by the CASS test and the Corrodkote test, and are substantially superior in corrosion resistance as measured by the H. A. Smith test and the I-Iuey test.

An alloy formed according to this invention is extremely well suited for forming, by deep drawing, articles such as ASTM designation B 368-61T. 2 ASIM designation B 380-611. 3 Corrosion Handboo H. H. Uhlig, 1948, p. 1022. 4 Corrosion Handbook, H. H. Uhlig, 1948, p. 1016.

cooking pots, metal caps for fountain pens and similar articles which are severely cold formed. This type of article, which may be deformed as much as 50% or more, can be drawn from an alloy according to this invention without forming more than 10% martensite.

Alloys of this invention can also be satisfactorily cold headed, and they also have good stress-rupture properties at elevated temperatures, thus making them well suited for bolts and valves and other articles subject to stress at elevated temperatures, such as automobile engine valves. Stress rupture testing on l-inch diameter bars of Heat 55744 hot rolled and annealed at l,850 F. for 15 minutes yielded the following data:

Extrapolating this data on a stress vs. Larson-Miller parameter (I TI 20+logt] 10 wherein P is the parameter, T is the temperature in 'R and t is time in hours as described in ASM handbook, 8th Edition, Vol. 1, page 472), the stress for producing l-percent plastic strain in 100 hours at l,350 F. is 4,350 p.s.i.

Tests have shown that valves formed by cold heading bars of this alloy exhibit excellent resistance to corrosion in internal combustion engines, and the other physical and chemical properties of the alloy are of such a nature that the alloy is well adapted for such use. These uses are illustrative, however, and they are by no means intended to limit the use of alloys of this invention, since the inherent desirable characteristics of the alloy suggest many varied uses.

Although several embodiments of this invention have been shown and described, various adaptations and modifications may be made without departing from the scope and appended claims.

What I desire to claim is:

1. A stable, austenitic stainless steel consisting essentially of, up to about 0.12% carbon, from about to about 8.5% manganese, up to about 2% silicon, from about 15 to about 17.5% chromium, from about 3.5 to about 6.5% nickel, from about 0.75 to about 2.5% copper, up to about 0.05% nitrogen, and the remainder essentially iron with incidental impurities, the steel being characterized by having good deep'drawability as the constituents of the steel are controlled so that the martensite-forming characteristic is less than 10 percent according to the formula A and which is further characterized by good hot workability as the constituents are further balanced to produce a delta ferrite-forming characteristic of less than 10 percent according to the formula delta ferrite potential Cr+l.5(% Si)0.87[30(% C+% N)+% Ni+0.5(% Mn)+0.3(% Cu)]+l, and wherein the amount of copper is controlled so that it does not exceed 3.85-0. l 8(% Mn).

2. A stable, austenitic stainless steel consisting essentially of, up to about 0.07% carbon, from about 5.5 to about 6.5% manganese, up to about 2% silicon, from about 16.1 to about 16.6% chromium, from about 5 to about 5.5% nickel, from about 1.5 to about 2% copper, up to about 0.05% nitrogen, and the remainder essentially iron with incidental impurities, the steel being characterized by having good deep drawability as the constituents of the steel are controlled so that the martensite-forming characteristic is less than l0 percent according to the formula martensite= 3582l(% C+% N)6.42(% Mn)l l.7(% Cr)l2.7(% Ni)22.7(% Cu), and which is further characterized by good hot workability as the constituents are further balanced to produce a delta ferrite-forming characteristic of less than 10 percent according to the formula delta ferrite potential=% Cr+l .5(% Si)-0.87 [30(% C+% N)+% Ni+0.5(% Mn)+0.3(% Cu)]+l, and wherein the amount of copper is controlled so that it does not exceed 3.5-0. 1 8(% Mn).

3. As an article of manufacture, a valve formed from a sta-' ble, austenitic stainless steel consisting essentially of up to about 0.12% carbon, from about 5 to about 8.5% manganese, up to about 2% silicon, from about 15 to about 17.5% chromium, from about 3 to about 6.5% nickel, from about 0.75 to about 2.5% copper, up to about 0.05% nitrogen, and the remainder essentially iron with incidental impurities, the steel being characterized by having good deep drawability as the constituents of the steel are controlled so that the martensiteforming characteristic is less than 10 percent according to the formula martensite 35822l(% C+% N)-6.42(% Mn)l l.7(%

Cr)l2.7(% Ni)22.7(% Cu),

and which is further characterized by good hot workability as the constituents are further balanced to produce a delta ferrite-forming characteristic of less than 10% according to the formula delta ferrite potential Cr+l.5(% Si)0.87[30(% C+% N)+% Ni+0.5(% Mn)+0.3(% Cu)]+l, and wherein the amount of copper is controlled so that it does not exceed 3.5-0. 1 8(% Mn).

4.- As an article of manufacture, a valve formed from a stable, austenitic stainless steel consisting essentially of, up to about 0.07% carbon, from about 5.5 to about 6.5% manganese, up to about 2% silicon, from about 16.1 to about 16.6% chromium, from about 5 to about 5.5% nickel, from about 1.5 to about 2% copper, up to about 0.05% nitrogen and the remainder essentially iron with incidental impurities, the steel being characterized by having gooddeep drawability as the constituents of the steel are controlled so that the martensite-forming characteristic is less than 10% according to the formula martensite 358-22l(% C+% N)-6.42(% Mn)l l.7(% Cr)l2.7(% Ni)22.7(% Cu), and which is further characterized by good hot workability as the constituents are further balanced to produce a delta ferrite-forming characteristic of less than 10% according to the formula delta ferrite potential Cr+l.5(% Si)0.87[30(% C+% N)+% Ni+0.5(% Mn)+0.3(% Cu)]+l, and wherein the amount of copper is controlled so that it does not exceed 3.5-0. 1 8(% Mn). 

2. A stable, austenitic stainless steel consisting essentially of, up to about 0.07% carbon, from about 5.5 to about 6.5% manganese, up to about 2% silicon, from about 16.1 to about 16.6% chromium, from about 5 to about 5.5% nickel, from about 1.5 to about 2% copper, up to about 0.05% nitrogen, and the remainder essentially iron with incidental impurities, the steel being characterized by having good deep drawability as the constituents of the steel are controlled so that the martensite-forming characteristic is less than 10 percent according to the formula % martensite 358-221(% C+% N)-6.42(% Mn)-11.7(% Cr)-12.7(% Ni)-22.7(% Cu), and which is further characterized by good hot workability as the constituents are further balanced to produce a delta ferrite-forming characteristic of less than 10 percent according to the formula delta ferrite potential % Cr+1.5(% Si)-0.87 (30(% C+% N)+% Ni+0.5(% Mn)+0.3(% Cu))+1, and wherein the amount of copper is controlled so that it does not exceed 3.5-0.18(% Mn).
 3. As an article of manufacture, a valve formed from a stable, austenitic stainless steel consisting essentially of up to about 0.12% carbon, from about 5 to about 8.5% manganese, up to about 2% silicon, from about 15 to about 17.5% chromium, from about 3 to about 6.5% nickel, from about 0.75 to about 2.5% copper, up to about 0.05% nitrogen, and the remainder essentially iron with incidental impurities, the steel being characterized by having good deep drawability as the constituents of the steel are controlled so that the martensite-forming characteristic is less than 10 percent according to the formula % martensIte 358-221(% C+% N)-6.42(% Mn)-11.7(% Cr)-12.7(% Ni)-22.7(% Cu), and which is further characterized by good hot workability as the constituents are further balanced to produce a delta ferrite-forming characteristic of less than 10% according to the formula delta ferrite potential % Cr+1.5(% Si)-0.87(30(% C+% N)+% Ni+0.5(% Mn)+0.3(% Cu))+1, and wherein the amount of copper is controlled so that it does not exceed 3.5-0.18(% Mn).
 4. As an article of manufacture, a valve formed from a stable, austenitic stainless steel consisting essentially of, up to about 0.07% carbon, from about 5.5 to about 6.5% manganese, up to about 2% silicon, from about 16.1 to about 16.6% chromium, from about 5 to about 5.5% nickel, from about 1.5 to about 2% copper, up to about 0.05% nitrogen and the remainder essentially iron with incidental impurities, the steel being characterized by having good deep drawability as the constituents of the steel are controlled so that the martensite-forming characteristic is less than 10% according to the formula % martensite 358-221(% C+% N)-6.42(% Mn)-11.7(% Cr)-12.7(% Ni)-22.7(% Cu), and which is further characterized by good hot workability as the constituents are further balanced to produce a delta ferrite-forming characteristic of less than 10% according to the formula delta ferrite potential % Cr+1.5(% Si)-0.87(30(% C+% N)+% Ni+0.5(% Mn)+0.3(% Cu))+1, and wherein the amount of copper is controlled so that it does not exceed 3.5-0.18(% Mn). 